Recently, as a memory used in electronic devices, there is a demand for a non-volatile memory operated at a high speed while consuming low power. As a currently used memory, there are a dynamic random access memory (DRAM), a flash memory, and the like, that use charge accumulation. The DRAM has been used as a main memory of a computer, but is a volatile memory of which the contents are lost when a power supply is turned off. Further, rewriting needs to be performed at a predetermined interval so as to maintain data even during the operation, such that power consumption increases. Meanwhile, the flash memory is a non-volatile memory, but a writing time of information is slow on the order of μs. It is expected to use a magnetoresistive random access memory (MRAM) as a non-volatile memory that does not have the above drawbacks, consumes low power, and is operated at a high speed.
FIG. 1A illustrates a basic structure of a magnetoresistive memory. The magnetoresistive memory includes a magnetoresistive element 103 that is disposed between a bit line 101 and a word line 102, and a transistor 104 for selecting each resistive element. The magnetoresistive element 103 has a structure in which an insulating layer 107 is interposed between a free layer 105 that is a magnetic layer of which the magnetization direction can be inverted by external magnetic field or spin injection and a fixed layer 106 that is a magnetic film of which the magnetization direction is in a fixed state, and as illustrated in FIG. 1A, when the magnetization directions (arrow in the drawings) of the free layer 105 and the fixed layer 106 are parallel with each other, an electric resistance of the magnetoresistive element 103 decreases, and as illustrated in FIG. 1B, when the magnetization directions of the free layer 105 and the fixed layer 106 are antiparallel with each other, the electric resistance of the magnetoresistive element 103 increases. A memory in which a difference in electric resistance of the magnetoresistive element 103 corresponds to a 1.0 signal is a magnetoresistive memory and manufacturing of the magnetoresistive element that is a core part is important. Therefore, if the electric resistance is Rp when the magnetization directions are parallel with each other and the electric resistance is Rap when the magnetization directions are antiparallel with each other, an element of which the magnetoresistance (MR) ratio represented by the following Equation is high has been developed.
                              MR          ⁢                                          ⁢          ratio                =                                            Rap              -              Rp                        Rp                    ×          100          ⁢                      (            %            )                                              [                  Equation          ⁢                                          ⁢          1                ]            
Further, in FIG. 1A, the magnetization directions of the free layer 105 and the fixed layer 106 may horizontally face a film surface, but may vertically face the film surface. In order to increase the MR ratio, a film structure or a manufacturing method of the magnetoresistive element has been developed, and S. Ikeda et al., “Tunnel magnetoresistance of 604% at 300K by suppression of Ta diffusion in CoFeB/MgO/CoFeB pseudo spin-valves annealed at high temperature” Appl. Phys. Lett. 93 (2008) 082508 discloses a result in which the MR ratio of 604% is achieved. Further, in addition to the magnetoresistive memory, a magnetic head, a magnetic sensor, and the like, using the magnetoresistive element has been developed rapidly. In the manufacturing of the magnetoresistive element, a technology of fine processing an insulating layer made of magnetic materials including elements, such as Fe, Co, Ni, and the like, that are used for the free layer or the fixed layer or magnesium oxide (MgO), aluminum oxide (AlO), and the like, by dry etching is required. As the dry etching method, a method of using ion beam etching and a method of using plasma etching have been used. In particular, the plasma etching has been widely used for the manufacturing of the semiconductor device, and it has excellent productivity in that a large diameter substrate may be etched uniformly. In addition, the plasma etching has characteristics of improving a selection ratio for various hard mask materials by using chemical reaction.
The plasma etching is carried out by introducing process gases into a decompressed process chamber and applying high frequency power (hereinafter, referred to as source power) from a source power supply to the process chamber via a flat antenna, a coil-shaped antenna, and the like, so as to generate a plasma of the process gases and to irradiate ions or radicals generated in the plasma to a substrate. An example of the plasma source may include several types, such as an effective magnetic field microwave type, an inductively coupled plasma (ICP) type, a capacitively coupled plasma (CCP) type, and the like, according to a difference in types generating plasma. Further, in order to positively attract ions in the plasma to a wafer, there is a case in which the high frequency power (hereinafter, referred to as wafer bias power) is applied even to a stage on which the wafer is disposed. As a method of processing a magnetic film using the plasma etching, the method using a plasma of Ar gas (K. Kinoshita et al. “Etching Magnetic Tunnel Junction with Metal Etchers” Jpn. J. Appl. Phys. 49 (2010) 08JB02.), the method using a plasma of mixed gas of CO and NH3 (Japanese Patent No. 02677321) and the method using a plasma of CH3OH gas (Japanese Patent No. 04111274) have been studied.
FIG. 2 illustrates an example of the method of processing a magnetoresistive element using the plasma etching. In FIG. 2, reference numeral 201 represents a Si substrate, reference numeral 202 represents an electrode film, reference numeral 203 represents an underlayer for controlling crystallinity of a fixed layer or stabilizing a magnetization of the fixed layer, reference numeral 204 represents a fixed layer, reference numeral 205 represents an insulating layer, reference numeral 206 is a free layer, reference numeral 207 represents a cap layer for protecting the free layer, reference numeral 208 represents a hard mask, and reference numeral 209 represents a resist mask. Although not illustrated in FIG. 2, transistors for selecting each resistive element or wirings for coupling each element are formed between the Si substrate 201 and the electrode film 202. Further, there is a case in which the underlayer 203 or the cap layer 207 is not present. As one of the processing methods of the magnetoresistive element, there is a method, as illustrated in the left drawing of FIG. 2, of forming each layer, etching the hard mask layer 208 and the cap layer 207 in the process (1) shown in FIG. 2, and etching only the free layer 206 in the process (2) shown in FIG. 2. In this method, it is particularly important to etch only the free layer 206 during the process of FIG. 2(2). K. Kinoshita et al. “Etching Magnetic Tunnel Junction with Metal Etchers” Jpn. J. Appl. Phys. 49 (2010) 08JB02 reports a result of etching only the free layer 206 using Ar plasma.